The present invention relates to an apparatus and a process of evacuating and mixing a mixture of substances or a preparation consisting of at least two components. In particular, the present invention relates to an apparatus and a method of degassing a polymer powder and of mixing the polymer powder with a monomer component in a mixing vessel from which the formed mixture of substances which is free of gas can be transferred into an application vessel.
The term "mixing" is generally understood to mean a preparation made up of two different substances. If one of the two substances to be mixed is a fluid and the other a powder, air is usually transferred and introduced into the initial preparation via the powder to which air particles can adhere. As a rule, even more air is introduced into the mixture by the mixing process per se.
On the one hand, the purpose of mixing is to achieve an as homogeneous distribution of the individual components as possible in the mixture, e.g., in order to initiate or promote an as uniform time-dependent chemical reaction as possible between the components, for example the subsequent polymerisation of a two- or multi-component mixture, such as bone cement. On the other hand, a mixture having as few inclusions or occlusions as possible, i.e. no impurities, is to be formed by mixing; for example, as few air inclusions as possible are to be included in the mixture. The less air bubbles are occluded, the higher is for example the strength of the prepared and hardened bone cement.
Several types of mixing apparatus and mixing processes are already known. Mixing can for example be effected by stirring, mingling, rolling, kneading, emulsifying, suspending, centrifuging and/or dissolving or by subjection to ultrasound or vibrators.
Most of the known mixing apparatus and processes have, however, the drawback that impurities, such as air, are introduced into the mixing system during the mixing process and, moreover, that the gas inclusions contained at the beginning and formed during the mixing process, for example due to a chemical reaction of the substances to be mixed, cannot be removed from the mixing system. Attempts have already been made to prevent the formation of gas inclusions by vacuum mixing systems, for example by means of the mixing apparatus described in WO 87/05492 or WO 88/09209 or the system described in the European Patent Application No. 85 11 3151.6.
In the known vacuum mixing apparatus and methods, however, certain air inclusions cannot be completely removed. This is in part due to the fact that the powder component of the mixture of substances to be mixed contains air and must initially be stirred into the liquid component before a vacuum can be applied since otherwise the powder would be sucked off during the evacuation step. Since then a certain reaction, e.g. a polymerisation, between the powder component and the liquid component of the mixture of substances has already started, the air cannot be completely removed any more.
On the other hand, attempts have already been made to evacuate the powder within the packaging, which is a method known for example from the foodstuff industry. Such a method has for example been proposed by Tepic and Perren (Bone cement preparation with vacuum packaged powder to minimize monomer content and increase strength, in: Draenert K. Die
Implantatverankerung, Symposiumband, pp. 26, 27, Art and Science, Munich 1985). The disadvantage of this system, however, resides in the fact that all spherical particles of the powder contact each other in a compact structure and are so closely packed that the monomer cannot embed the individual spherules quickly and sufficiently enough to form the desired uniform sphere packing, like exposed aggregate concrete for example; the resulting structure is rather comparable to that of lean concrete.
It is an object of the present invention to provide an apparatus and a process of vacuum mixing a mixture of substances, by means of which the existence of impurities, in particular gas inclusions, in the mixture of substances can be avoided.